Emperically, thermals can go from gopher farts to funnel clouds and CuNim in the lower 3-4,000 ft of the atmosphere.

I have bits and pieces of a plane left behind by such a thermal at Cal Valley two years ago. It went from totally visible to invisible in a single turn. 90 degree flaps and crow, right stick in the bottom corner and the vario was still screaming the affirmative. Fun while it lasted, which was all of 30 seconds.

Seeyou has some analysis functions that can histogram the strength of thermals recorded by altitude bands ... if I remember, I'll grab my data from work tomorrow and post a few of the plots. If I recall correctly, several of these kinds of trends are pretty visible from the data. We have a couple years' of data from Cal Valley and Montague, so we should be able to quantify the "typical" improvement with altitude, as well as the distribution of lift at our typical altitudes. Sorry for the no-data-post, but this way I'm subscribed and have a higher chance of remembering ;-)

A few competing things here. High up flown correctly has very few turns if any. Conversely in my case if I am super high and don't fly well it is anything but a straight line. Flying high takes a lot more concentration and discipline.

Been looking at gps data and comparing planes to each other given different lift scenarios. Who wins depends entirely on what assumptions you make about lift rate and sink rate of the air. This is all using min sink and max l/d. Perhaps speed to fly will begin to differentiate the designs. Long way from showing any results.

I'm going to post this even though it is a bit premature because I want people to challenge the assumtions. It's not giving me the answer I want and I want to see if it can be so or not. I compared 4 planes, #8, XCBD, SBXC, and #7 which had a DS21/19 airfoil and 14" root. I did an analysis of a 50 mile course with the following assumptions:
Lift 3 times the sink rate
Thermal spacing 3 miles or 4827m boomer day, 1 mile on inverted day
Thermal strength not a function of altitude
Inversion layer 500m present or not
Polars for the individual planes were calculated using the following method, and I will admit this is shaky:
Plot the SBXC polar data from Dan Edwards paper, actual data.
Write a formula that scales and offsets the much better XFLR5 data to the actual measured data.
Apply this formula to the 3 other polars from XFLR5 to get an approximation of a "real" polar for the other 3 planes.
Graph the polars and calculate the speed to fly from the tangent to the polars.
Final glide at speed to fly through sink.
Max altitude based on angular visibility dimension of the chord, 13" goes to 1500m.
All planes 5 kilos.

On the inverted day with ultralight lift, the #7 wins because it has the lowest sink rate which dominates over the speed.
On the inverted day with good lift, the XCBD wins because it has the highest speed to fly and the flattest glide slope. Small difference in sink rate is negligible.
On the good day with 3m/s lift and 1m/s sink, the XCBD still wins even though it is capped at 1211m vs #7 at 1615m, though by only 3 minutes in a 130 minute race.

This suggests that the only way to justify a larger chord is to make the assumption that lift is stronger at a higher altitude. I have seen GPS evidence both for and against this. I have also had runs and I am sure other people have also where once I got "on top" there seemed to be more lift than sink and it was relatively easy to run a long time without losing much altitude. I can't explain this one but I have seen it on occasion on the GPS.

Next I will throw the giant Supra in the mix and see what camber change does.

I grabbed a selection of flights from people who completed the tasks or at least had really long flights. At first appearance, nearly all the flights (Mayo 2008, Cal Valley 2009, Montague 2009) support the "stronger thermals with higher altitude" and give a curve also.

However, I think we would be foolish to use this data verbatim. We spend more time in the lower bands since we both climb up and sink back down through these regions. The high altitudes typically we climb through only once or twice on boomers and sink back through quickly as the pilot gets aggressive. So I think I'd say to temper the upper 1/3rd by some percentage you feel appropriate.

I'll let the data do the rest of the talking. I am quite happy to send the full analysis by SeeYou for anyone wanting a particular flight or two.

I added the giant Supra to the spreadsheet. The short of it is on the light lift inverted day it beats everything because it can camber up and climb faster. On the rest the XCBD still wins but not by much, 3 minutes on the 130 minute race. This is all with the assumption that thermal strength does not vary with altitude. Tomorrows fun will be to dig through Dan's data and think about how to modify the model for varying lift with altitude. That may change things.

I'm not claiming that the spreadsheet is free from errors, and the scaling from XFLR5 to try to match the real data from the tests is still suspect, especially in the case of the Supra. That said, if we assume that the lift and associated sink are 1.4X stronger above the XCBD proposed ceiling of 1211m, then the 13.25" root chord #8 wins. The scores get very close. I think the 1.4X factor is more than fair based on the GPS data and personal observations, and the stronger the lift is above the cutoff the more it favors the chord. Interesting that after all this work I was not far off with the last design. I'll want some stick time on the Supra but it is looking like I need to design the best possible package that fits in a 13" root 160" span plane.

OK time for a new approach to the problem. This time I will take one design, say #8 which is not bad, is fully public, and has lots of miles on it. Now take the root design and scale the spans up and down. Then scale the chords up and down. Scale the ceiling according to the visibility angle. Use the unaltered XFLR5 polars since we are strictly comparing now, not trying to match results to real plane performance. Now race the planes against each other in different conditions and see which one comes out on top. This should narrow down what size and shape plane to build, then the design will fit in that space.

Will, I think you will be very happy with the plane. I keep racing it in the computer and it keeps winning. To beat it you have to go very high, higher than most people are willing to fly. If you have never flown anything this big you will be amazed at the performance.

So by this model I am flying the right sized plane. The difference is not much but these races are now won by minutes and even seconds. Of course one bad decision wipes out any advantage. What this tells me is clipping the span on a good day doesn't hurt much if you have a good planform because you trade L/D for wing loading. Adding span slows the plane down too much because you lose too much wing loading, at least with this airfoil set. Cutting chord costs you a little because you can't go as high in the big lift. Adding chord costs you a little in both wing loading and aspect ratio. Interesting what I come up with is pretty close to a Wiley sized plane, either stock or with clipped wingtips.

Now comes the fun part, packing the most efficient plane inside the size package that the analysis tells me.

So by this model I am flying the right sized plane. The difference is not much but these races are now won by minutes and even seconds. Of course one bad decision wipes out any advantage. What this tells me is clipping the span on a good day doesn't hurt much if you have a good planform because you trade L/D for wing loading. Adding span slows the plane down too much because you lose too much wing loading, at least with this airfoil set. Cutting chord costs you a little because you can't go as high in the big lift. Adding chord costs you a little in both wing loading and aspect ratio. Interesting what I come up with is pretty close to a Wiley sized plane, either stock or with clipped wingtips.

Now comes the fun part, packing the most efficient plane inside the size package that the analysis tells me.

Greg, this is interesting, especially the 14" chord

after last weekend I am convinced that I want 14" for visibility and resulting ability to fly higher safely. Seems to me 14" chord and 150" could result in a very fast racer with wingloading around 16 oz. But how much would it lose in low reynolds, high AoA thermalling performance? I imagine full span camber changing is now a part of the ultimate compromise design?

Camber change is definitely on the table. Next step is to go through the library of airfoils to see which root I want to start with. I will also run the 14" x 150" to see the effect of high chord and boosting the wing loading.